On Twitter, the passing along the tweet of another with some degree of disapproval. It can range from strong (that’s a lie) to mild (there are exceptions or conditions). It shouldn’t be taken as impolite or unfriendly - it’s done in the spirit of twitterville - as a new way to participate in the conversation. Done by adding 'DeTweet' or 'DT' before repeated post.
I thought his tweet was untrue, so I'm DeTweeting it to let others know they shouldn't believe it.